1. Field of the Invention
This invention relates to the electric range art and particularly to metal-sheathed electrical resistance heating elements for use with flat plate surface heating units and glass-ceramic plate cooktops.
2. Description of the Prior Art
Metal-sheathed electrical resistance heating elements of coiled configuration are widely used for the cooktops of electric ranges. See the Prucha/Bowling U.S. Pat. No. 3,767,897, which is assigned to the present assignee.
Smooth surface glass-ceramic cooktops have become very popular for use on both electric and gas ranges. See the Siegla U.S. Pat. No. 3,612,828 which shows a single-plate, utensil-supporting, glass-ceramic cooktop having a plurality of open coil heating elements positioned therebeneath to provide several areas for surface cooking. These open coil heaters are supported by fibrous insulation pads. In this system, the main mode of heat transfer between the open coil heater and the glass-ceramic cooktop is by radiation since the heater is vertically spaced from the cooktop by an air gap. In order to produce high radiant heat to obtain acceptable heating rates, the heater coil is operated at relatively high temperatures on the order of 1800.degree. to 2000.degree. F. at a wattage rating of about 2000 watts at 236 volts AC. This presents several problems. The open coil heater is in direct contact with the insulation pad, and in other commercial designs it is partially embedded into the insulation. Hence, the insulation pad operates at a very high temperature. This causes a considerable heat flow downwardly to overheat the rough-in box, as well as high heat losses, and contributes to a relatively low thermal efficiency on the order of 50 percent. Also, only a high quality, expensive insulation can be used at such high operating temperatures. Moreover, an open coil heater may be exposed, in the event the glass-ceramic were to break unintentionally. Open coil heater systems are also expensive, on the order of more than twice as expensive as standard metal-sheathed electrical resistance heating elements.
An earlier design of glass-ceramic cooktop using a metal-sheathed heating element is shown in the Dills U.S. Pat. No. 3,632,983, which is also assigned to the present assignee. This Dills patent shows a glass-ceramic cooktop including a shallow mounting or rough-in box that contains a filler plate that has recesses for accommodating the heating units and wiring raceways for containing the electrical lead wires.
The principal object of the present invention is to provide a glass-ceramic cooktop with a higher-efficiency, lower-cost, lower thermal mass, metal-sheathed heating element assembly.
A further object of the present invention is to provide a metal-sheathed heating element assembly of the class described with a molecular film diffusion barrier that protects the glass-ceramic plate from reacting with the sheath material without creating an appreciable resistance to the heat flow from the sheath to the plate.
A further object of the present invention is to provide a metal-sheathed heating assembly of the class described with means to reduce the heat losses in a downward direction from the heating element so as to improve the thermal efficiency of the heating element.